Enhanced core stability, defined as the ability to maintain or resume an equilibrium position (or trajectory) of the trunk (and pelvis) after perturbation, has been touted to improve injury-prevention in many sports; however, this causal relationship has not been critically tested in novice runners, a group often plagued by injuries. Running has been employed by millions of people in the US to battle obesity and maintain a healthy lifestyle, yet injuries such as patellofemoral pain (PFP) and iliotibial band syndrome (ITBS) often force many runners to quit the activity all together. Studies focused only on the site of injury (knee, foot, etc.) have yielded little change in the prevalence of injuries. In addition, approximately half of the body's mass resides in the upper body, and control of this mass will impact the forces received by the lower extremities. The lack of understanding of the role that the torso and pelvis play in running represents a critical barrier to progress in the creation of paradigms to prevent running injuries. Our global hypothesis is loss of core stability increases the forces and moments in the lower extremities that are known to increase the risk of PFP and ITBS in runners. In this proposal, we will test the role of core stability in novice runners by (1) developing a within session core stability knockdown protocol that reduces the ability to control movement of the torso and hips by 25% without fatiguing the muscles of the lower extremity, and (2) identifying the changes in PFP- and ITBS-relevant loading at the knee and hip during running that occur due to the core muscle knockdown protocol developed in Aim 1. Currently, a major limitation of studies investigating running injuries is the reliance on long-term, multiple vsit studies that are often hampered by high drop-out rates. To accomplish this research, we will develop a novel within-session core muscle exercise protocol to reduce, or knock down, core stability. Running biomechanics will then be evaluated before and after this protocol in 25 novice runners to determine how knee and ankle loads associated with PFP and ITBS are influenced by a loss of core stability. The innovation of this project is in the application and expansion of existing methodologies in measuring core stability and lower extremity biomechanics to a novel application: investigating the mechanism by which core stability influences biomechanical predictors of lower extremity injury risk. Moreover, we will develop a novel core muscle knockdown protocol to establish this mechanism. Lastly, this application sets the stage for long-term studies to compare the effectiveness of interventions for the prevention of running-related injuries. The results of this project will significantly contribute to characterizing how the loss f core stability contributes to running injuries, and the novel techniques that will be developed in this work could contribute to future biomechanics studies.